Molecular dynamics approach for controlling triaxial stress states and its application to void evolution in nano-grained polycrystals

This study presents a novel implementation scheme for accurately controlling constant triaxial stress states, specifically stress triaxiality T and Lode parameter L, in molecular dynamics simulations. The scheme has broader applicability for exploring deformation and damage mechanisms in nanocrystalline materials under complex triaxial stress fields, including metals, structural alloys, and composites. As an example, the evolutions of both intragranular and intergranular voids in nano-grained polycrystals have been simulated using this approach, with special emphasis on the effect of triaxial stress states on nanovoid growth and the associated intrinsic physical mechanisms. The results show that under certain triaxial stress states, neither dislocation emission from the nanovoid surface nor plastic deformation of surrounding grains can completely accommodate the rapid nanovoid growth. A distinct dominant void growth mechanism, i.e., the void-surface expansion and propagation along grain boundaries intersecting such a nanovoid surface, has been proposed and discussed. This mechanism may dominate the nanovoid evolution behavior at high stress triaxialities T≥2, regardless of the applied Lode parameter L. Furthermore, the initially spherical nanovoid can evolve into cylinder-like shape as observed in experiments under the moderate stress triaxiality T=1. In addition, it is intriguingly found that for all the considered stress triaxialities T={0.375∼3}, the plastic deformation and ultimate failure of materials may be insensitive to the pre-existing intragranular nanovoid with its diameter less than a critical value Dc. With increasing the stress triaxiality T, the void-insensitivity critical size initially decreases abruptly and, finally, achieves a stable value.